Absolute Cell Reference.

A spreadsheet term.

In a spreadsheet you can refer to another cell in a formula by using a 'cell reference' such as A1. This refers to column A, row 1. The simplest formula would look like

=A1

Spreadsheets also allow you to drag that formula down or across or even copy it to another cell. Normally the formula you end up with in each cell is different. For example the next cell down would have =B1

If you want to stick to the same cell, regardless of how you drag or copy it then you use an 'Absolute cell reference' like this

=$A$1

The $ tells the spreadsheet not to alter the formula as you drag or copy it to another cell.